An Alpha Research exit poll said GERB won 32.2 percent of the vote, with the Socialist Party coming in second with 28 percent. A separate exit poll by Gallup International Balkan had GERB with 32.8 percent and the Socialists with 28.4 percent.

Official results are expected Monday.

The election had been seen as a test of Bulgaria's loyalties to the European Union and to Russia, with which it has historic political and cultural links. Bulgaria is set to take over the bloc's presidency in 2018.

GERB and the Socialists both campaigned to revive economic ties with Russia to benefit voters in the European Union's poorest nation. But the Socialists vowed to go further, even if it meant upsetting the country's European Union partners.

GERB did not win enough votes to govern alone, and will likely form a coalition government with the United Patriots, an alliance of three nationalist parties that the exit polls showed placing third.

The Socialists ruled out any option of serving in a coalition government. But Ninova said her party would look at options for forming a government should the center-right GERB party find it cannot do so on its own.

Borisov, 57, resigned as prime minister after his party lost the presidential election last year. Parliament was dissolved in January, and the president appointed a caretaker government that will stay until a new government is formed.